The present invention relates to UHF high power (100kw and greater) broadcast transmitting antenna systems and particularly to such a system for transmitting television broadcast channels 21 to 70, operating at 100 kw and greater and in which the transmission line between the transmitter and radiating antenna includes a substantial length of circular waveguide.
Broadcast antennas for television are frequently located on top of high towers that are constructed to offer minimum wind resistance. The transmitter is located on the ground near the tower and the radiating antenna is located at the top of the tower. A transmission line from the transmitter to the antenna is carried by the tower, usually at the outside of the structure and is attached thereto by hardware that includes hangers, ties, etc. The cost of the tower, the transmission line, its hardware, elbows, transitions, transformers, barriers, hangers, ties, etc., constitute just about the total cost of the antenna system. That cost, the cost of maintenance and the cost of operation are all important considerations. The cost of operation is directly determined by the efficiency of the transmission line from the transmitter to the antenna (everything else being equal).
Heretofore, UHF (120 to 1000 mhz) high power broadcast TV antennas for channels 21 through 70 have used rectangular waveguide rather than coaxial transmission line for conducting the RF power from the transmitter up the tower to the radiating antenna. Waveguide is preferred to coaxial line (coax), because the waveguide is more efficient, it costs less, it does not have an inner conductor and at the higher power requirements for UHF, in a six to nine inch diameter coaxial line, multi-moding occurs and this is undesireable. The only disadvantage of rectangular waveguide is that it offers a greater wind load and so the tower has to be constructed sturdier.
Due particularly to the higher order multi-moding and the undesireability of that moding, the Electronics Industry Association (EIA) has recommended that the upper limits of the use of coaxial transmission line be as set forth in the table below which is a comparison of the relative costs of coaxial and rectangular waveguide recommended by EIA for the UHF TV channels 26 through 70:
______________________________________ Coax line Rectangular Waveguide Channel Size $/Ft. Size $/Ft. ______________________________________ 70 61/8-50 76 WR - 1150 65 50 8 3/16-75 123 WR - 1150 65 38 8 3/16-50 133 WR - 1500 79 37 9 3/16-75 143 WR - 1500 79 26 9 3/16-50 151 WR - 1500 79 ______________________________________
Clearly, the cost difference becomes quite significant for a one thousand foot tower and far exceeds the slight increase in costs of the supporting tower structure to accomodate the greater wind load of the rectangular waveguide over the coaxial transmission line.
However, many old towers are marginal in regard to the ability to withstand wind load and simply can not use rectangular waveguide, because of the added wind load. This has given rise to the use of circular instead of rectangular waveguide. The advantages of circular over rectangular waveguide are: lower wind load, greater efficiency, lower installation costs, no tortional twists and no rotation due to manufacturing twist. The disadvantage is that: unless the circularity of the circular waveguide along its entire length is maintained within very precise tolerances, the polarization of waves launched into the guide in the dominent TE11 mode is accompanied by a transverse polarization and by undesireable higher modes like the TM01 and so the efficiency of the coupling from the transmitter to the circular waveguide and from the circular waveguide to the antenna suffers. It is an object of the present invention to provide a method and means of overcoming these problems with circular waveguides.